This invention relates generally to power supply devices transmitting AC input electrical power to loads sensitive to loss or large variations in the AC input supplied electrical power. It particularly relates to such devices supplying standby electrical power to sensitive loads upon loss or large variation in the AC input electrical source. These devices generally are described as uninterruptible power supplies or UPS devices.
A UPS device generally operates in one of two modes: normal and standby In the normal mode, the UPS device transmits electrical power from the AC input supply to the sensitive load while measuring the AC input source for losses or large variations. Upon measuring or determining a loss or a large variation in the transmitted electrical voltage greater than the load can withstand, the UPS device forward transfers to the standby mode to supply electrical power to the load from a battery. This maintains the supply of electrical power to the sensitive load for continued operation of the load or for the load to turn off in an orderly sequence. In the standby mode, the device measures the AC input source of electrical power for reverse transferring to the normal mode upon proper return of the AC input source.
The load could be such as a desk top computer that needs electrical power to retain its operating data and programs A sudden over or under voltage or complete loss of power can effect a loss of data that is expensive to re-assemble or, at least, an annoying interruption and restarting of a software program. Other devices such as medical life support or life monitoring devices likewise need constant supplies of electrical power for proper operation. These same general principles of normal and standby mode operation and forward and reverse transfer apply to large and small loads and large and small UPS devices
One known on-line manner of effecting a UPS device constantly rectifies the AC input power to DC and then inverts the DC to AC to within closely regulated voltage limits. Some of the intermediate DC power maintains the charge in a battery that supplies the electrical power upon an under voltage or loss of power from the line source. Over voltages become absorbed in the intermediate DC stages of the UPS device. This kind of operation continuously rectifies and inverts, with attendant power losses, the AC input electrical power that normally occurs within acceptable limits. This continuous operation of the rectifier and inventer components in the normal transmission of line electrical power ages those components much more quickly than if they were operated intermittently only in the standby mode, raising reliability problems Also the rectifier and inventer components can produce loud, continuous audible noise.
A second known switched manner of effecting a UPS device uses a relay or other mechanical switching device to connect the AC input source to the load in normal transmission of electrical power and to disconnect the AC input from the load and connect a battery powered standby inventer to the load upon loss or large variation of the AC input electrical power. This type of system, in the normal mode, fails to provide the sensitive load with isolation from potentially harmful AC input source events such as voltage spikes or surges or to provide the load with voltage regulation. Transfer between the two modes occurs slowly and with unwanted electrical noise because of the mechanical limitations of the switch. Common-mode and normal mode noise attenuation usually is minimal.
The last or triport known manner of effecting a UPS device uses a three port ferroresonant transformer. Normal transmission of electrical power occurs loosely from a first primary winding to a secondary winding through a high reactive impedance of the transformer. The AC input connects to the first primary winding while the load connects to the secondary winding. In standby mode, the utility connection opens and standby electrical power becomes loosely supplied from a battery through an inventer to a second primary winding of the transformer and therefrom through the high reactive impedance of the transformer to the secondary winding and the load. The two primary and one secondary windings form the three ports of this type of UPS device.
While providing voltage regulation, the ferroresonant transformer is heavy and inefficiently transmits power to the load while causing a high output impedance. Transfer between modes occurs quickly, but special timing is needed to synchronize the inventer phase with the output phase to avoid producing output voltage spikes, especially with highly capacitive loads.
These three known types of systems separately embody several previously known UPS device characteristics that until now could not have been provided in one UPS device. These include efficient transmission of line power in a normal mode of operation, light weight, inherent reliability of circuit arrangement and component usage, low output impedance especially in the normal mode, regulation of the AC input voltage transmitted to the load and fast transfer between normal and standby modes with minimal or an absence of electrical noise at transfer. Further desirable features include attenuation of common-mode and normal-mode AC input source noise and isolation of the load from the AC input source.
Known output voltage regulation to the load for AC input voltage variations occurs in an on-line UPS device by regulating the AC voltages produced by the output inventer. See for example U.S. Pat. No. 4,410,935 to Dang. This regulation is substantially non-existent in switched devices and triport devices provide voltage regulation by controlling the magnetic saturation of the iron core of the ferroresonant transfer. See for example U.S. Pat. No. 4,400,624 and U.S. Pat. No. 4,475, 047.
Known transformers used in UPS devices consist mainly of ferroresonant, high reactive impedance transformers used in triport devices. These transformers insert magnetic shunts between the first primary winding and the secondary winding and between the second primary winding and the secondary winding. Such an arrangement provides for constant operation of the standby inventer at a particular phase relative to the AC input phase for the standby inventer to act as a rectifier of electrical power from the transformer to charge the battery. By changing the standby inventer phase, this circuit supplies electrical power to the load through the transformer, after the AC input becomes disconnected from the first primary winding. See for examples U.S. Pat. No. 4,475,047; U.S. Pat. No. 4,400,624; U.S. Pat. No. 4,238,691 and U.S. Pat. No. 4,238,688.
Alternatively, the high reactive impedance transformer can have the two shunts dividing the iron core into three sections with the core carrying the output secondary winding on a terminal section and the first and second primary windings on the other two sections. See U.S. Pat. No. 4,556,802. This arrangement of windings supposedly provides for parallel supply of power by the standby inventer circuit during normal mode operation and the elimination of a switch to disconnect the line source upon transfer to standby mode operation.
Ferroresonant transformers inherently are large and heavy and poorly or loosely couple electrical power from input to output. This is used to advantage in previous UPS devices to obtain AC input voltage regulation but remains an overall disadvantage from a device system view. Tightly coupled power or isolation transformers are much smaller, lighter and efficient, but additional circuits must be used for regulation. Further, most standby inverters synthesize the desired 60 hertz AC sinusoidal output waveform by pulse width modulation (PWM) techniques driven through an inductor. The loose coupling of a ferroresonant transformer readily furnishes this desired inductance. A power transformer does not supply this inductance and requires an additional inductor in the secondary output circuit to integrate the PWM signal and obtain the sinusoidal waveform output.
Known UPS devices of the switched or triport type require some phase locked loop circuit to synchronize the phase of the standby power with that of the line source to continue smooth transmission or supply of electrical power to the load at forward transfer from normal to standby mode and at reverse transfer from standby to normal load. Phase synchronization avoids producing voltage spikes to the load and the UPS device supplying or absorbing instantaneous quantities of power at transfer.
Many such phase-locked loop circuits are known. For example U.S. Pat. No. 4,638,176 discloses phase-locked loop 70, in a UPS device, producing a square wave reference at an undisclosed phase-shifted angle.
What is desired thus is a UPS device that efficiently and with regulation transmits AC input electrical power to a sensitive load during normal operation and that reliably supplies the load with standby electrical power of the same AC phase upon loss or large variation of the AC input source. The UPS device should isolate the load from the AC input, attenuate common-mode and normal-mode input noise, provide a low output impedance and transfer between modes without electrical noise while continuously supplying or transmitting electrical power to the load.